Chiropractic care is a widely used form of manual therapy focused on the body’s musculoskeletal and nervous systems. The primary therapeutic tool is the spinal adjustment, also known as spinal manipulation. This technique involves applying a controlled, sudden force to a specific joint in the spine or extremities. This physical action on the joint structure produces a physiological response, using hands-on pressure to improve function and reduce discomfort.
The Biomechanical Goal of Spinal Adjustment
The immediate purpose of a spinal adjustment is to address restricted joint movement, often called joint fixations. When a spinal joint loses its normal range of motion, surrounding tissues can become irritated and stiff. The chiropractor applies a high-velocity, low-amplitude (HVLA) thrust to the joint, pushing it beyond its passive range but within anatomical limits. The goal is to restore the joint’s movement and improve the alignment of the spinal segment.
This rapid separation of joint surfaces often produces an audible “pop” or “crack,” known as cavitation. Cavitation occurs when the sudden drop in pressure within the joint capsule allows dissolved gases, such as nitrogen and carbon dioxide, to rapidly escape and form a bubble. This sound is the release of gas from the synovial fluid that lubricates the joint, not bones cracking or grinding together. While the sound is a common byproduct, the clinical effectiveness of the adjustment is not dependent on hearing the cavitation.
Neurological Pathways and Pain Reduction
The most significant mechanism of a spinal adjustment involves its direct influence on the nervous system. Joints, muscles, and tendons are richly supplied with sensory nerve endings called mechanoreceptors and proprioceptors. These receptors sense motion, pressure, and position of the body. The quick, controlled force of the adjustment intensely stimulates these nerve fibers.
The stimulation of these large-diameter nerve fibers (1A and 1B afferent fibers) is the foundation for the Gate Control Theory of Pain. This theory posits that the spinal cord contains a neurological “gate” that regulates the flow of pain signals to the brain. Pain signals travel along smaller, slower nerve fibers, while the rapid, non-painful input from the adjustment travels along larger, faster fibers.
By flooding the nervous system with this high-speed information, the adjustment effectively “closes the gate” in the dorsal horn of the spinal cord. This sensory overload interrupts or inhibits the transmission of slower nociceptive signals. Consequently, the brain registers less pain, even if the source of discomfort remains. This immediate nerve response is the primary physiological mechanism for the pain relief many people experience after an adjustment.
Systemic Effects on Muscle Tone and Inflammation
Beyond the immediate sensory interruption of pain, the neurological response leads to broader, systemic effects on surrounding tissues. Restoring proper joint motion and improving nerve signaling helps normalize the reflex arc between the joint and associated muscles. When a joint is restricted, surrounding muscles often enter a state of hypertonicity, presenting as chronic tension or spasms.
The adjustment helps break this cycle by sending improved proprioceptive signals, causing a reflex relaxation of tense muscles. As muscle tension decreases, localized pressure on nerves and blood vessels is reduced, contributing to comfort. This muscular relaxation is a direct effect of the neurological input from the adjustment.
Furthermore, mechanical and neurological changes can help moderate the body’s inflammatory response. Localized joint restriction and muscle hypertonicity can create a low-grade inflammatory state. By restoring normal movement and reducing nerve irritation, localized chemical irritants are minimized. Some research suggests spinal manipulation may influence the immune system by reducing pro-inflammatory chemical messengers, such as cytokines like TNF-α and IL-6.